Certain liquids and gases transported in railway tank cars or tank trucks are particularly hazardous and at elevated temperatures may expand within the tank and increase the internal pressure to a dangerous level. Government regulatory agencies therefore require operators of these vehicles to install safety pressure relief valves which are set to open automatically above a specified pressure level and vent at a specified rate of discharge.
Presently employed pressure relief valves are generally biased to the closed position such as by a coil spring or a constant-force spring. Coil spring valves usually require the pressure in the tank to continue to increase beyond the initial opening pressure in order to reach the maximum venting capacity. The constant-force spring valves, on the other hand, have the advantage of opening to a maximum venting capacity instantly at the initial opening pressure without any further increase in pressure. Notwithstanding this advantage, size, complexity and cost of the constant-force spring valve have mitigated their general acceptance in the industry. Space restrictions and design constraints also limit the number of springs to only a few making the valve more difficult to "tune" for a precise initial opening pressure, or to maintain the valve element aligned with the valve seat for positive closure.